Seismic Slosh: Swimming Pools and Fish Tanks.

I’ve blogged before about reservoirs where I considered their ultimate hazard, -- complete dam failure. Other, less catastrophic hazards associate with them too. One is seismic slosh. “Slosh happens”  -- I’m told --  whenever a standing body of fluid gets shaken from below. Recall that fancy dinner party when soup was just served? Someone (I still claim that it wasn’t me!) bumped the table leg and many bigwigs watched bouillabaisse elope from their bowls. 

A similar slosh happens during earthquakes. Instead of an errant blogger knocking a table leg, seismic shaking does the deed. I’ve seen several You Tubes where, during a quake, an amateur cameraman captured seismic slosh in a backyard pool or living room fish tank.

OK, now imagine that you are in charge of a hilltop urban reservoir a million times larger than a back yard pool. Upon sensing an earthquake you leave your office and witness oscillations not just a few inches, but many meters form in the basin. Within a minute, slosh overtops the reservoir walls. You know that that more is at stake here than lost drinking water -- sloshes form a real hazard to the site.  One might toss flows several meters deep from storage and you have many critical facilities nearby -  pumping houses, power transformers, chemical treatment shacks. An unplanned flood could knock these out for weeks.  Moreover, a powerful overland flow can undermine roads, erode walls and maybe trigger the ultimate hazard considered in that previous blog.  Even if none of that happens, where does that surge go? Down hill certainly, to the nearest creek, canyon, or storm drain. More than wet napkins, reservoir slosh has potential for flash flooding folks living below.

I’m no expert, but I created a computer simulation that illustrates the subject. The movie shows a flat-bottomed reservoir dimensioned 700 m by 900 m, 50 m deep with 10 m high walls subject to just two 30-second shaking cycles of less than 10% g acceleration. Now, 10% g is quite modest shaking, just verging on damaging. Nevertheless, the motion parents slosh with vivid effectiveness. Water 6 m deep escapes the reservoir to places unknown. 

Scientists understand seismic slosh and they employ simulations like these to mitigate its effects in the reservoir design stage. If they do their job right, slosh hazard will be limited to swimming pools and fish tanks.

Steven N. Ward    Santa Cruz